where /another/ghc is a path to another local GHC repository. You can specify a remote repository here too, e.g. -r http://darcs.haskell.org/ghc-6.10 (remember to omit the final "ghc" when using a remote repo).

The darcs-all command is useful for finding out what patches you have relative to another repository:

$ ./darcs-all -r /another/ghc push --dry-run

this tells you which patches there are in your local repository tree relative to the tree over in /another/ghc.